Preparation and photocatalytic activity of WO3-MWCNT nanocomposite for degradation of naphthalene under visible light irradiation

Article abstract of DOI:10.1039/c6ra04642h

In this study, a WO₃-multiwalled carbon nanotube nanocomposite has been prepared for the first time by in situ liquid phase process. The prepared nanocomposite was used for photodegradation of dilute solution of naphthalene under visible light irradiation. Based on our results, comparing photocatalytic activity of WO₃ nanoparticle with WO₃-multiwalled carbon nanotube nanocomposite showed that the photodegradation of naphthalene is negligible by using pure WO₃ nanoparticles while, composition of WO₃ nanoparticles with multi walled carbon nanotubes could improve significantly their photocatalytic activity under visible light. Due to its high electrical conductivity, carbon nanotube can transfer photogenerated electron on its surface and in this way decreases electron-hole recombination rate and increases photocatalytic activity. After the reaction, the irradiated solution has been analyzed by gas chromatography and mass spectrometry techniques for identification of the naphthalene photodegradation intermediates and products. 1-Naphthol, 1,4-naphthalenedione and 1,2-benzendicarboxilic acid have been determined as intermediates and based on these intermediates a suitable mechanism for photodegradation of naphthalene was suggested.

Full text of DOI:10.1039/c6ra04642h

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Preparation and photocatalytic activity of WO3-MWCNT
nanocomposite for degradation of naphthalene under visible light
irradiation
Received 00th January 2016,
Accepted 00th January 2016
Mousa Farhadian,a Parvaneh Sangpour*a and Ghader Hosseinzadehb
DOI: 10.1039/x0xx00000x
In this study, WO3-multi walled carbon nanotube nanocomposite has been prepared for the first time by in-situ liquid phase
process. The prepared nanocomposite was used for photodegradation of dilute solution of naphthalene under visible light
irradiation. Based on our results, comparing photocatalytic activity of WO3 nanoparticle with WO3-multiwalled carbon
nanotube nanocomposite showed that the photodegradation of naphthalene is negligible by using pure WO3 nanoparticles
while, composition of WO3 nanoparticles with multi walled carbon nanotubes could improve significantly their
photocatalytic activity under visible light. Due to its high electrical conductivity, carbon nanotube can transfer
photogenerated electron on its surface and in this way decreases electron-hole recombination rate and increases
photocatalytic activity. After the reaction, the irradiated solution has been analyzed by Gas chromatography and Mass
spectrometry techniques for identification of the naphthalene photodegradation intermediates and products. 1–naphthol,
1,4-naphthalenedione and 1,2-benzendicarboxilic acid have been determined as intermediates and based on these
intermediates a suitable mechanism for photodegradation of naphthalene was suggested.
www.rsc.org/
band gap (3.2eV for anatase and 3eV for the rutile structure),
TiO2 based photocatalysts are active only under the UV light
irradiation.20 Since only about 4-5% of the solar radiation falls in
1. Introduction
Polycycilic aromatic hydrocarbons (PAHs) are chemical
pollutants with two or more benzene rings. Incomplete
combustion at high temperature (500–800 °C) or exposure of
organic materials to low temperature (100–300 °C) for long
periods results in PAHs production.1 They are emitted from
fossil fuels, by-products of industrial process and pyrolysis of
organic materials and are often considered to be mutagenic and
the UV range 21-23, for better use of this free source of radiation,
it is necessary to shift the adsorption region of photocatalyst
semiconductors toward longer wavelengths or use of another
semiconductors with suitable band gap which absorb
irradiation in the visible light region. WO3 nanostructures are
considered as a semiconductor with suitable band gap (2.4-2.8
eV) for absorption of visible light, in addition to narrow band
gap, its harmless and stability in aqueous media over a wide
range of pH are other advantages of this compound.24, 25 But
fast recombination of the photo-generated electron-hole limits
its photocatalytic activity.26 There are some ways for resolving
these restrictions such as: deposition or doping of noble metals
carcinogenic material.2, Naphthalene, the first member of
3
PAHs group and one of the 16 PAHs has been warned as a
dangerous pollutant by Environmental Protection Agency (EPA)
and International Agency for Research on Cancer (IARC).4
Many techniques such as bioreactors,5 ozonation,6-8
photodegradation,9-13 and adsorption14-16 have been applied
successfully for removing of PAHs from wastewater of industrial
plants and soils. Because of low cost, not using of toxic materials
and other merits of photodegradation method, in this study,
this method was used for removing of naphthalene from water.
TiO2 nanostructures are the most used photocatalyst in the field
of photodegradation of organic pollutants and recently have
been used for photodegradation of the PAHs pollutants under
Ultraviolet (UV) irradiation.2, 17-19 But because of its unsuitable
27-29
or metal ions30 in its structure, composition with other
semiconductors,31, 32 modification of physical properties such as
their particle size,33 and engineering its morphology.34,
35
Recently, composition of WO3 with carbon nanostructures is
considered, because of increasing electron-hole recombination
lifetime and improving the efficiency of photocatalytic
activity.36-38
Multi walled carbon nanotube (MWCNT) has unique electronic
and mechanical properties such as high surface area, high
electrical conductivity and so on. Therefore, decorating of base
semiconductor such as WO3 nanoparticles (NPs) with MWCNT
could improve the photocatalytic activity of WO3 by significant
increasing in surface area for adsorption, and remarkably
decreasing in the electron-hole recombination rate.39, 40 The
MWCNT decorated WO3 NPs have been used for degradation of
a. Nanotechnology and Advanced Materials Department, Materials and Energy
Research Center (MERC), Karaj, Iran
b. Nanobiotechnology Research Center, Baqiyatallah University of Medical Science,
Tehran, Iran
* Corresponding Author: E-mail: Sangpour@merc.ac.ir; Tel:+98-26-36280053;
Fax:+98-26-36204139
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azo dyes by other researchers.41 But, there is a few study of precipitate was washed 3 times with deionized water to remove
View Article Online
DOI: 10.1039/C6RA04642H
excess ions. Then this precipitate ultrasonically was dispersed in
using this composite for degradation of PAHs compounds.
In this study, WO3-mullti-walled carbon nanotube deionized water and the ammonia solution was added to this
nanocomposite has been synthesized, and was used as a solution until pH 9 to form transparent solution of ammonium
photocatalyst for photodegradation of naphthalene in aqueous tungstic. A certain amount of Functionalized MWCNTs (f-
solution under visible light irradiation. The photodegradation MWCNT) was ultrasonically dispersed in mixture of
rate of naphthalene was studied using HPLC. With using Gas H2O/C2H5OH (v/v, 50/50) solvent. Under sonication, the
chromatography (GC) and Mass spectrometry (MS) techniques, ammonium tungstic solution was added drop wise in to the
intermediates
photodegradation were characterized and based on these formed after 2 h. The resulted product was centrifugally
intermediates suitable mechanism was proposed for separated and dried at 80 °C. Finally, to obtain the WO3-
photodegradation of naphthalene on WO3-MWCNT MWCNT nanocomposite, the product was calcinated at 500 °C
and
products
of
the
naphthalene dispersed f-MWCNT solution. The uniform suspension was
a
nanocomposite as photocatalyst.
for 2h.
2.5. Catalyst Characterization
Different techniques were used for characterization of the
nanostructures. X-Ray Diffraction (XRD) was performed on the
Philips X`Pert MPD Pro X-ray diffractometer with Co-Kα
irradiation (λ=1.54018 A˚). UV–vis spectroscopy (UV-vis
spectrometer (JASCO V-570)) was used to record the UV-vis
absorption spectra. The morphology and microstructure of
products investigated by means of Scanning Electron
Microscopy (SEM) (VEGA-TSCAN-XMUS teroscan) and
Transition electron microscopy (TEM) (Philips CM-200). FTIR
spectra were recorded by Brucker V33 FT-IR spectrometer using
the KBr Pellet technique. Brunauer Emmett Teller technique
(Belsorp II, mini Japan) was used for investigation of shape and
pore size distribution and also surface area.
2. Experimental
2.1. Chemicals and reagents
MWCNTs with 15-20 nm in diameter and 50-100 μm in length
with purity of >95%, were purchased from Nanostar Company
(Germany). Sodium tungstate dihydrate (Na2WO4.2H2O),
Ethanol, Nitric acid (65%), Hydrogen peroxide (35%), Oxalic acid,
Ammonia solution (37%), and naphthalene were obtained from
Merck (Germany).
2.2. Synthesis of WO3 NPs
The WO3 NPs were synthesized by precipitation method from
aqueous solution of sodium tungstate dihydrate. In a typical
method, 2.5g of the tungstate salt was dissolved in 100 ml of
deionized water and in the following the concentrated nitric
acid was added drop wise without stirring until pH<1. The
resulting yellow tungstic acid (H2WO4) precipitate washed
several times with deionized water to remove any extra ions
and then was redissolved in 50 ml of oxalic acid dehydrate
solution (0.3 M) to form clear solution. This solution was dried
at 100 °C and then was calcinated at 500 °C for 2 h.
2.6. Photocatalytic activity and photodegradation intermediates
determination
In this work, as an irradiation source, a 570 W Xenon lamp
(OSRAM Co) was used and L41 UV filter (Kenko Co.) was
employed to remove UV light (λ < 400 nm). Photocatalytic
reaction suspension was prepared by ultrasonically dispersing
of 50 mg of the as-prepared photocatalyst (WO3 and WO3-
MWCNT) in a 100 ml (naphthalene) NPH solution (10 ppm). This
suspension was magnetically stirred for about 2 h in dark
condition to establish adsorption-desorption equilibrium. After
this step, this solution was irradiated under visible light with
constant stirring and in a given time interval 5 ml of solution was
withdrawn and centrifuged. For keeping the temperature of
solution at 25 °C, water circulation was used around the
reaction container (Fig. 1). Similar to previous related
researches,43, 44 for photocatalytic activity measurements, the
remaining concentration of NPH in the withdrawn samples was
determined using HPLC ((LC-6A, Shimadzu) which were
equipped with an RP-C18 column (5 μm×125 mm×4mm) and a
UV–Vis spectrophotometer detector Model SPD-6AV. Using a
mixture of acetonitrile and water (60:40) isocratic eluent with
flow rate of 1ml/min, the naphthalene peak was appeared at
retention time of 6 min. The naphthalene absorbance was
monitored at wavelength of 276 nm and its peak area was
converted to concentration by using standard curve (obtained
from standard solutions of naphthalene). For intermediates
identification, NPH was extracted from the samples by 30 ml of
chloroform (3 × 10 ml) and analysed by Gas Chromatography
(GC) (6890 N-Alilent) and Mass Spectroscopy (MS) (5973-
2.3. Functionalizing of MWCNT
Before using, the MWCNTs were purified with HNO3 to remove
extra catalysts that employed in production process. After
purification, the MWCNTs were functionalized with mild acid
treatment including HNO3 and H2O2 solutions. For the oxidative
treatment, 0.3 g of agglomerated MWCNT was dispersed in 70
ml of HNO3 solution (3.0 M) by sonication and then was stirred
for 15 min at 60 °C. Then this slurry was filtered and washed
with deionized water to remove the excess acid from the
solution. For completing the oxidation process, the solution was
exchanged with H2O2 (v/v, 30%) and the procedure was
identically repeated.42
2.4. Synthesis of WO3-MWCNT nanocomposite
In this work WO3-MWCNT nanocomposite with various 1, 2, 4,
6, 8 and 10 w% was prepared which these composites named
as WO-CNT-1, WO-CNT-2, WO-CNT-4, WO-CNT-6, WO-CNT-8
and WO-CNT-10, respectively. For synthesis of the WO3-
MWCNT nanocomposite with different MWCNT content, a
certain amount of Na2WO4.2H2O was dissolved in deionized
water and then nitric acid was added drop wise to form the
yellow tungstic acid (H2WO4.2H2O) precipitate. The as prepared
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these compounds were automatically identified byVtiehweANrtiIcSleTOMnlinSe-
DOI: 10.1039/C6RA04642H
search software.
3. Results and Discussion
3.1. Characterization
FT-IR
FT-IR technique was used to characterize the oxygen chemical
bands and type of the functional groups grafted on the MWCNT.
Fig. 2a, shows the FT-IR spectrum of functional MWCNT (f-
MWCNT). As shown in this figure, the adsorption band at 3454
cm-1 is attributed to O–H stretching vibration of carboxyl group
(–COOH). Two characteristic peaks at about 1643 cm-1 and 1741
cm-1, could be assigned to the carbonyl and carboxyl groups
which bonded to the aromatic ring. The band at 2920 cm-1 is
assigned to asymmetric –CH2 stretching vibration. The broad
peaks at 1180 cm−1 could be assigned to C–O stretch from
phenol or lactones groups; and also to C–C bands. The band at
1643 cm-1 from carbonyl group and the band at 1730 cm-1 from
carboxyl group are ascribed to C=O stretching vibration. The
band at 1585 cm-1 from aromatic ring is attributed to the
stretching vibration of conjugated C=C bond.37, 45, 46 The FT-IR
spectrum of WO3 NPs is shown in Fig. 2b. Fig. 2c shows the FT-
IR spectrum of WO3-MWCNT composite, W–O characteristic
peak at around 712 cm-1 has been shifted to the longer
wavenumber (798 cm-1) due to the composition with f-MWCNT
and also, C=O band has been shifted toward shorter
wavenumber. So, formation of nanocomposite, containing WO3
NPs and f-MWCNT could be confirmed.47
Fig. 1 Schematic illustration of the photoreactor setup.
Alilent) with a capillary column (HP-5 ms). In GC-MS analysis,
the column temperature was held at 50°C for 2 min, then
increased to 300°C with ramp of 10°C/min, and finally held at
this temperature for 37 min. Electron Ionization mass spectra
were identified using NIST 2002 Library (Wiley) and most of
XRD
XRD technique was used in order to determine the crystalline
structure of the MWCNT, WO3 NPs, and WO3-MWCNT
Fig. 3 X-ray diffraction pattern of (a) MWCNT, (b) WO3
Fig.
2 FT-IR spectrum of (a) f-MWCNT, (b) WO3
nanoparticles, and (c) WO3-MWCNT nanocomposite.
nanoparticles, and (c) WO3-MWCNT nanocomposite.
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nanocomposite which are shown in Fig. 3, respectively. The formed and low percentage of the WO3-MWCNT
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three characteristic peaks in Fig. 3a are related to the (002), nanocomposite is in hexagonal phase.37, 41
DOI: 10.1039/C6RA04642H
(100), and (004) diffracting planes of the MWCNT.46 These peaks
SEM and TEM images
disappear during the compositing of MWCNT with WO3 NPs.
The reason is that these peaks in the nanocomposite are
overlapped by the peaks of the WO3 NPs, or the presence of
WO3 NPs peaks might lead to mask of the characteristic peak of
the MWCNT at 2θ about 25.8°.47 Phase converting of WO3
occurred according below:41
The microstructure and morphology of nanocomposite were
characterized with SEM and TEM techniques (Fig. 4). As Fig. 4a
shows the MWCNTs are nearly monodisperse and have high
aspect ratio. Surface morphology of the MWCNTs doesn’t
remarkably change during the formation process of WO3-
MWCNT nanocomposite (Fig. 4b). presence of WO3 NPs on the
surface of MWCNT could be confirmed semi-quantitatively by
Energy-dispersive X-ray (EDX) spectroscopy analysis (Fig. 4c), as
could be seen from this figure three main component of are C
(5.6%), O (27.3%), and W (69.07%) elements (more detailed
analysis of the chemical state(s) of these elements coms in XPS
section). Fig. 4d shows TEM images of WO3-MWCNT
nanocomposite. The sample has been consisted of WO3 NPs and
Therefore, only orthorhombic phase can form at 500 °C. All of
the peaks appeared in Fig. 3b could be attributed to the
orthorhombic crystalline structure of WO3. But, in Fig. 3c, which
is XRD pattern of WO3-MWCNT nanocomposite, in comparison
with Fig. 3b some of peaks such as (111) peak were disappeared
which shows the orthorhombic phase was not completely
Fig. 4 (a) SEM image of MWCNT, (b) SEM image of WO3-MWCNT nanocomposite, (c) EDX pattern of the WO3-MWCNT
nanocomposite and (d) TEM image of as prepared WO3-MWCNT nanocomposite.
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N2 Adsorption-Desorption Isotherms
MWCNTs components which the WO3 NPs are bonded to the
surface of MWCNTs. The mean diameter of the MWCNTs is
about 15nm and the WO3 NPs are approximately 40nm in
diameter.
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To investigate surface area and porosity DoOf IW: 10O.130N39P/sC6aRnAd04W64O23H-
MWCNT nanocomposite nitrogen adsorption-desorption
isotherms was used. The results are illustrated in Fig. 6. For WO3
NPs, the isotherm is a combination of the type ІІІ and V isotherm
parents with H3 hysteresis loop in the range of 0.4–0.9 P/P0,
Optical properties
Optical properties of nanostructures were studied by UV-vis indicating the existence of weak interaction between adsorbent
spectrophotometer compatible for solid sample analysis. Fig. 5, and adsorbate according to the IUPAC classification (Fig. 6a).52
demonstrates the optical adsorption spectra of WO3 NPs, and After composition with MWCNT, the hysteresis loop shifts to
WO3-MWCNT nanocomposite. The adsorption edge of WO3 NPs higher relative pressure (P/P0) ranges and the areas of the
at 440 nm was shifted to a longer wavelength (475 nm) after hysteresis loop become smaller, which indicating increasing of
compositing with MWCNT. The red shift of the adsorption edge average pore-size and decreasing of average pore-volume (Fig.
results in a reduction of band gap energy and also a decrease in 6b). The corresponding pore-size distribution of WO3 NPs and
the photoinduced electron-hole recombination rate, and hence WO3-MWCNT nanocomposite was determined by using the
increases photocatalytic activity under visible light.48 The Barrett-Joyner-Halenda (BJH) method from the desorption
optical absorption coefficient (α) of a semiconductor is branch of the isotherm. The WO3 NPs show features of micro-
expressed by the following equation (1):49
ꢆ
ꢈ
α = ꢀ ꢁℎꢂ − ꢃꢄꢅ ꢇꢈℎꢂ
(1)
Here α is the absorption coefficient, Eg is the absorption band
gap, A is a constant depending on the transition probability, n
depends on the nature of the transition, i.e., allowed direct,
allowed indirect, forbidden direct and forbidden indirect. In our
case, for an indirect band gap semiconductor of WO3, the value
of n is 2. Assuming the materials to be indirect semiconductor,
a plot of modified Kubelka–Munk function versus the energy of
incident light50 for the WO3 and WO3-MWCNT nanocomposite
results in the band gap energies (Eg) of 2.87 and 2.68 eV for
these materials, respectively. The decrease in the band gap can
be attributed to the formation of W–C and W–O–C bands in the
MWCNT-WO3 nanocomposite. Recently, similar results for the
CNT-doped TiO2 has been reported by O. Akhavan and et al.51
Fig.
5 Absorption spectra and Tauc plot (inset) for
Fig. 6 Nitrogen adsorption/desorption isotherms and BJH
pore-size distribution curves of (a) WO3 nanoparticles and
(b) WO3-MWCNT nanocomposite.
measurement the band gap energy of WO3 and WO3-
MWCNT samples.
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XPS
and mesoporous materials, whereas combination of it with
MWCNTs, the microporous features decrease and the
mesoporous features increase. The calculated average pore
diameter and BET surface area of the WO3 NPs and WO3-
MWCNT nanocomposite were measured about 1.85 nm and 40
m2.g-1, 1.64 nm and 160 m2.g-1, respectively.
Increasing the effective surface area of photocatalyst has a vital
role in decreasing of mass transfer limitation of pollutant to its
surface so the photocatalytic activity of photocatalyst could be
improved with increasing of its surface area. As obtained results
show compositing of WO3 NPs with MWCNT in WO3-MWCNT
nanocomposite remarkably increases the effective surface area
of this photocatalyst (40 m2/g for WO3 NPs versus 160 m2/g for
WO3-MWCNT) and in this way could increase its photocatalytic
activity.
View Article Online
DOI: 10.1039/C6RA04642H
The surface composition and chemical states of elements of the
as prepared WO3-MWCNT nanocomposite were further
investigated using X-ray photoelectron spectroscopy. Fig. 7a
represents XPS survey spectrum of WO3-MWCNT
nanocomposite, demonstrating that the nanocomposite
contains W, C, and O. Fig. 7b shows the high-resolution XPS
spectra of the W 4f region. The binding energy of W 4f7/2 and W
4f5/2 core levels are observed at 35.68 eV and 37.85 eV which
are attributed to the energies of the doublet of W6+.53 Their
highly symmetrical shape indicates that W is only in the
oxidation state of +VI. For further analysing the chemical stat of
C, the high-resolution XPS spectra of the C 1s region was
examined (Fig. 7c). Deconvolution of the broad asymmetric C 1s
peak demonstrates that carbon elements of the nanocomposite
have been consisted from four types of carbon including sp2-
Fig. 7 (a) XPS survey spectra of WO3-MWCNT , (b-d) high-resolution XPS spectra of the W 4f, C 1s and O 1s region of WO3-
MWCNT, respectively.
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hybridized, hydroxyl (C-OH), carbonyl (C=O) and carboxyl (O- This model is generally used for the photocatalytic degradation
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C=O) carbon in binding energies of 284.5, 285.7, 288 and 289 eV process when the initial concentration of the pollutant is low.
respectively.54 This results confirm successful surface functional Fig. 8b depicts the ln(C/C0) versus the irradiation time for
of MWCNT. Presence of such surface group at surface facilitates different as prepared photocatalysts. WO3 nanoparticles
WO3 binding to the surface MWCNT. Again deconvolution of the presents an apparent reaction rate constant k of 0.0014 min-1
high-resolution XPS spectra of the O 1s region in Fig. 7d shows under the irradiation of visible light. While, WO-CNT-4 showed
oxygen element presence in different chemical state which the maximum reaction rate constant (0.0067 min-1), which is 4.8
bonded to W, H and C elements in different functional groups.
times that of WO3. Other reaction rate constants using the
different photocatalysts under visible light (λ > 400 nm) are
summarized in table 1.
3.2. Photocatalytic activity and photodegradation intermediates
study
Because of high effective surface area and excellent electrical
conductivity, MWCNT was introduced to the WO3 nanoparticles
structure in order to increase the surface area, reduce the
electron-hole recombination rate, and finally improve the
photocatalytic activity. At first with increasing the MWCNT
contents in the WO3-MWCNT nanocomposite, the
photocatalytic activity increases but more MWCNT addition has
destructive effect. Since the accumulation of these types of
carbonaceous material at surface of base photocatalyst (WO3)
acts as an obstacle for incident light, more MWCNT loading
could lead to a significant decrease of photocatalytic activity.
Clearly, when the MWCNT contents exceed from 10 wt%, the
photocatalytic activity of the WO3-MWCNT nanocomposites is
deteriorated. Although, MWCNT is beneficial for decreasing the
photogenerated electron-hole recombination rate of WO3-
MWCNT photocatalyst, but it can shade WO3 nanoparticles in
too much weight percentages content. Therefore, the WO-CNT-
4 with 4 wt% of MWCNT has the optimum photocatalytic
activity, and the higher content of MWCNT could be detrimental
to the photocatalytic efficiency (notice that in all other parts of
this paper only this optimum photocatalyst was used and
referred as WO3-MWCNT nanocomposites). Due to the balance
between charge separation and light harvesting, the
photocatalytic performance of WO3-MWCNT nanocomposites
at first increases and then decreases with increasing of the
MWCNT content.
Photocatalytic activity
The adsorption of pollutant molecules on the surface of
photocatalyst is a prerequisite for good photocatalytic activity.
Table 1 displays the remaining percentage of NPH in solution
after reaching the adsorption-desorption equilibrium (2 hour)
over the as-prepared photocatalysts in dark condition. Due to
the increasing the active surface area, with increasing the
MWCNT contents, the adsorption capacity of the
photocatalysts increase.
Fig. 8a shows time profiles of C/C0 under visible light irradiation,
where C is the concentration of NPH at the irradiation time of t
and C0 is the concentration in the adsorption equilibrium of the
photocatalysts before irradiation. The results show that under
visible light irradiation, the photocatalytic activity of the bare
WO3 nanoparticles was very low because of the high
recombination rate of photogenerated electron-hole pairs at its
surface. Surface modification of WO3 nanoparticles by
compositing with MWCNT, transfer photogenerated electron-
hole pairs to MWCNT surface and by this way reduce the
electron-hole recombination rate and increase photocatalytic
activity.
The pseudo first order kinetic model was used for
determination of reaction rate constant (k) of NPH
photodegradation which expressed by Eq. (2):
ln ꢉꢊꢊ ꢌ = −ꢍꢎ
(2)
ꢋ
Fig. 8 (a) Time profiles of C/C0 and (b) The corresponding pseudo-first-order kinetics model for the photocatalytic activity of
different photocatalysts for NPH degradation (under radiation with λ>400 nm).
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GC/MS Analysis
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DOI: 10.1039/C6RA04642H
Table 1. Reaction rate constant remaining percentage of
NPH after the adsorption equilibrium for different
photocatalysts.
In our research, GC analysis of sample after 4 hour of reaction
revealed a series of peaks at different retention times (tR) from
5 to 25 min. the results show the slight decomposition of NPH
Photocatalyst K (min-1)
remaining percentage
of NPH after the
adsorption equilibrium
Table 2. The identified intermediates in photocatalytic
degradation of naphthalene at the recent researches.
WO3
13.7×10-4
40.6×10-4
58.5×10-4
64.0×10-4
54.2×10-4
48.0×10-4
33.7×10-4
96%
91%
89%
86%
84%
83%
83%
photocatalyst
Source
Intermediates
Ref.
WO-CNT-1
WO-CNT-2
WO-CNT-4
WO-CNT-6
WO-CNT-8
WO-CNT-10
High
&low
1-naphthalenol
1,4-naphthalenedione
alkyl phthalates
17
TiO2
pressure
Hg lamp
1-naphthalenol
1,4-naphthalenedione
2-Formylcinnamaldehyde
1-naphthalenol
125W
Hg
12
TiO2
Because of its π-electron conjugation system and high electron
conductivity, MWCNTs can reduce the recombination rate of
the photogenerated electron-hole pairs in its embedded
photocatalyst. Furthermore, due to the good dispersion of the
WO3 nanoparticles on the MWCNTs surface, composition of the
WO3 nanoparticles with MWCNTs can decrease the
agglomeration of the WO3 nanoparticles and by this way
increases the light absorption cross section of the nanoparticles.
The other important role of MWCNTs is the increasing of the
effective surface area of the base semiconductor which by
increasing the diffusion of pollutant molecules from solution to
the active sites of the photocatalyst, results in the increasing of
the physical adsorption of pollutants on the surface of
nanocomposite and therefore effectively improvement of the
photodegradation performance of the photocatalyst. MWCNTs
can also decrease the band gap energy of the WO3
nanoparticles anchored on its surface and result in the light
absorption shift to the longer wavelength. However when the
contents of MWCNTs in the composite exceed from the
optimum value they act as a barrier against the incident light
and the photocatalytic activity of the photocatalyst was
deteriorated.
1,4-naphthalenedione
1,2-benzenedicarboxylic
acid
1500-W
Xe
55
TiO2
1,2-
benzenedicarboxaldehyde
3 -phenyl-2 -
propenaldehyde
5,8-dihydroxy-1,4-
naphthoquinone
125 W
Hg
2,5,8-trihydroxy-2,3-
dihydronaphthaquinone
10
TiO2
4,4,5,8-tetrahydroxy
anthraquinone
1,2-
benzenedicarboxaldehyde
2H-1-benzopyran-2-one
1,4-naphthalenedione
1-naphthalenol
In order to ascertain the active species (hvb+, OH• and O2
•–
radicals) responsible for the photocatalytic activity of WO-CNT-
4, three different scavengers were added to the reaction
2
TiO2
UV-A
UV-A
suspension: KI as an hvb scavenger, methanol as an OH•
+
scavenger and benzoquinone as an O2 scavenger.55 As Fig. 9
•–
2-Hydroxy-1,4-
naphthalenedione
shows in presence of KI decreasing in photocatalytic activity of
WO-CNT-4 is negligible so hvb plays no role in the
+
1,2-benzenedicarboxylic
acid
photodegradation of NPH by this photocatalyst. However, in
presence of methanol and specially benzoquinone the
photocatalytic activity of WO-CNT-4 remarkably decreases.
Since in presence of the mixture of methanol and benzoquinone
scavengers the photocatalytic activity of WO-CNT-4 nearly
vanishes therefore O2•– radicals together with OH• radicals play
the main role in the photocatalytic degradation of NPH by WO3-
MWCNT photocatalyst.
13
α-Fe2O3
Phthalic anhydride
1-naphthalenol
WO3-
MWCNT
570 W-
Xe
1,4-naphthalenedione
This
work
1,2-benzenedicarboxylic
acid
8 | RSC Adv., 2016, 00, 1-11
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ARTICLE
For comparison with this work, the previous studies on the
View Article Online
photodegradation of NPH and also deteDctOioI:n10o.1f03in9t/Cer6mRAe0d4i6a4t2eHs
using different photocatalyst in aqueous media have been
summarized in the Table 2. For example; TiO2 is photoactive in
the UV region and has been widely used for decomposition of
PAH compounds under UV source, but in our study the visible
light source has been used for photodecomposition of NPH.
Reaction Pathway
Dewar’s reactivity number (Nu) reflects the localization energy
and hence reactivity of a particular position on PAHs.56 The Nu
value of position 1 (1.81) is smaller than that of position 2 (2.12),
which means position 1 is more easily attacked by reactive
hydroxyl or superoxide species (Fig. 11). In this study, the main
product or intermediate was found to be 1-Naphthalenol (II)
which formed from the reaction of OH• radicals with position 1
•–
of NPH and subsequent O2 radicals with position 2 of NPH,
then releasing of hydroperoxyl radical (HO2•–) results in the
formation of 1-Naphthalenol as main product. The same
procedure is repeated and 1,4-Naphthalenediol (III) is created.
In the following, attacking of OH• radicals to position 2 of 1,4-
Naphthalenediol (III), releasing of H2O molecule, attacking of
Fig. 9 comparison the photocatalytic activity of WO-CNT-4 in
absence and presence of different scavengers.
•–
•–
O2 radical to position 4, and finally releasing HO2 radicals
result in the formation of 1,4-Naphthalenedione (IV). In the
alternative procedure, O2•– radicals attack to position 1 of NPH
and finally results in formation of 1,2-Benzenedicarboxylic acid
(VI). This reaction accompanied by releasing of acetylene (C2H2),
which results in the opening of one aromatic ring. Therefore,
to 1-Naphthalenol at tR of 14.4 in presence of WO3
photocatalyst and irradiation of visible light, but for
decomposition of NPH in presence of WO3-MWCNT
photocatalyst three intermediates (or products) have been
detected at tR of 9.9, 13.15 and 14.4 min in its GC chromatogram
(Fig. 10a-c). According to MS spectra the peaks observed at tR of
9.9, 13.15, 14.4 and 18.27 min could be attributed to
Naphthalene, 1,4-Naphthalenedione, 1-Naphthalenol and1,2-
Benzenedicarboxylic acid, respectively which 1-Naphthalenol
make the most portion (Fig. 10d-f).
•–
the OH• and O2 species are responsible for decomposition of
NPH. The plausible reaction mechanism is shown in Fig. 12.
Fig. 10 (a-c) GC chromatogram of NPH photodegradation by blank, WO3 and WO3-MWCNT under visible light, respectively and
(d-f) MS spectra of 1,4-naphthalenedione, 1-Naphthalenol and 1,2-naphthalenedicarboxilic acid intermediates respectively.
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4. Conclusions
View Article Online
DOI: 10.1039/C6RA04642H
The WO3-MWCNT nanocomposite was successfully synthesized
with in-situ method and was used as a high active photocatalyst
for degradation of NPH in visible light irradiation. Photocatalytic
degradation of naphthalene with WO3 NPs and also WO3-
MWCNT nanocomposite were investigated and compared. The
results of photocatalytic activity test showed that WO3 NPs have
a weak photocatalytic activity under visible light irradiation
because of high recombination rate of photogenerated
electron-hole pair. While, composition of WO3 NPs with
MWCNTs can improves its photocatalytic activity under visible
light by increasing its effective surface area and specially
reducing the recombination rate of photogenerated electron-
hole pair by transferring of the electron to conductive surface
of MWCNT. The irradiated solution after the photocatalytic
reaction has been analysed and 1,4-Naphthalenedione, 1,2-
Benzenedicarboxylic acid, and 1-Naphthalenol were detected as
intermediate or products. The intermediates produced in
photodegradation process identified by GC/MS. The reaction
mechanism of naphthalene photodegradation was proposed,
assuming the hydroxyl and superoxide radicals attack to the
position 1 and position 2 of naphthalene according to the Dewar
Number, respectively.
Fig. 11 (a) Numbering and (b) Nu (Dewar) values of NPH.
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View Article Online
DOI: 10.1039/C6RA04642H
Preparation and application of the novel and high performance WO3-MWCNT photocatalyst for
photodegradation of naphthalene under visible light irradiation.